1. Field of the Invention
This invention relates to apparatus for protecting residential and industrial electrical power installations from damage due to transients and surges that greatly exceed the maximum instantaneous line voltage. Such transients and surges can result from lightning, from switching occurring in the utility network, and from switching of inductive loads in the installations.
Low energy transients often have very fast rise times and peak voltages of 10,000 volts and higher. Even though the energy is low, the high voltages can damage common household equipment using low-level transistors, such as calculators, radios, and the like, and industrial equipment such as computers, instruments, and controls. Higher energy transients may involve energy as high as 100 joules and surge currents to 20,000 amperes. Inductive loads, such as motors and transformers, can experience buildup of high instantaneous voltages across the first few turns of their windings from such transients. Resultant arcing across the turns damages the insulation, resulting in shorting out of the turns. Even though failure may not be immediate, the decreased back emf causes increased power drain and overheating of the windings. Such effects are often progressive, resulting in the ultimate failure of the motor or transformer.
A major manufacturer of appliance motors has analyzed many failures and has determined that over 75% were directly or indirectly caused by transients or overvoltage surges. Motors can be designed to withstand surges; however, the extra cost is too great for the consumer appliance market.
Power and communications utilities fully recognize these problems and invest heavily in protection of their own equipment. However, most homeowners and many other power users are not aware of the transient and surge problem, and therefore accept a few years' life for motors and equipment, whereas these units should last indefinitely.
2. Description of the Prior Art
A wide variety of devices, filters, and techniques have been developed for protection of power lines, communication lines, motors, transformers, and electronic equipments from the damaging effects of transients and surges. A common approach for protection from high energy lightning transients is the use of spark gap devices such as gas discharge arrestor devices, spark gaps, and carbon blocks. While effective in shunting high currents to ground, these units have a relatively slow reaction time. Thus, fast rise time transients can reach dangerously high voltages before breakdown of the device occurs.
Zener-type semiconductor devices are available for protection against fast rise time transients; however, only small amounts of energy can be handled and these devices are easily damaged. Recently, metallic oxide varistors have been developed that can handle larger amounts of energy than the zener-type device, but the normal delay in breakdown is still excessive for high rise time transients.
In the manufacture of high-quality electronic and computer equipments, surge and transient filters are commonly built in. In most cases, filters are in series with the load, and must carry the full load current. Failures of such filters, therefore, would interrupt operation of the equipment.
No known prior art has been found that is suitable for protecting a home or building from damage from all types of transients and surges present on power lines, and that will not interrupt the power service upon failure.